1. Field of the Invention
The present invention relates to a method and an apparatus for forming a continuous oriented structure of a polymer.
2. Description of the Related Art
As the degree of integration of electronic circuits has increased, lithographic technology has reached its limits. The bottom-up approach, in particular, molecular devices composed of molecules, has attracted much attention. According to the bottom-up approach, a good inorganic conductor and/or semiconductor pattern can be formed on a substrate in one step, which is difficult to achieve using the existing lithographic technology. The bottom-up approach is highly compatible with large-area substrates and allows the use of substrates, such as plastic substrates, which have excellent impact resistance but poor heat resistance. The bottom-up approach also enables the use of non-flat substrates. Therefore, the bottom-up approach can lower manufacturing costs and provide higher device fabrication flexibility.
According to the bottom-up approach, polymers of good organic conductors and/or organic semiconductors are fundamental materials for molecular device fabrication, and extensive studies have been conducted on such polymers. For example, Japanese Patent Laid-Open No. 2005-328030 teaches a linear macromolecule of a porphyrin compound serving as a conductive molecular material.
Moreover, if polymers of the good organic conductors and/or organic semiconductors are soluble, these polymers can be arranged into desired shapes on substrates by a technique similar to a printing technique. One of the examples of the method for applying a solution of an organic semiconductor to form a desired shape is a method that includes forming a lipophilic region and a lipophobic region on a substrate, applying the solution of the organic semiconductor in an organic solvent over the entire surface of the substrate, and crystallizing the solution only in the lipophilic region. Other examples include a method employing printing technology and a method employing ink jet technology.
In order for the molecular device to achieve peak performance, the molecules or assemblies of molecules constituting the molecular device should be oriented. The process of forming an oriented structure is very important. One typical process for making an oriented structure of an organic polymer is to process a substrate in advance to impart it with the capability of controlling the orientation and then to form an oriented polymer structure on the substrate, e.g., a substrate-rubbing technique, a substrate-grating technique, or the like. Other examples include techniques for forming the oriented structure by applying an external force during formation of an assembly of a polymer, e.g., an electrical field-induced orientation technique, a magnetic field-induced orientation technique, a flow-induced orientation technique, and an epitaxial growth technique. Still another example is a process, such as a stretching-rolling technique, a frictional transfer technique, and an optical orientation technique, in which the external force is applied to achieve the desired orientation after the polymer is formed as an unoriented structure. For example, Japanese Patent Laid-Open No. 8-47971 teaches a super-strong packing band including a laminate of polyolefin films uniaxially oriented by a stretching-rolling technique. Yet another orientation technique involves transferring a polymer film spreading on a water surface onto a substrate (Langmuir-Blodgett technique).
These various orientation techniques have their own advantages and an appropriate technique is selected according to the characteristics of the polymer material and the purpose of the orientation. Whereas the orientation techniques described above involve soluble or meltable polymers, Japanese Patent No. 2535780 teaches a process of making an oriented thin film by sliding an insoluble, unmeltable polymer against a substrate.
Orienting the molecules by these orientation techniques is not only effective in fabrication of conductive molecular devices, but also very important in making various active polymer devices with optical and/or thermal anisotropy. Japanese Patent Laid-Open No. 2001-81202 teaches a sheet having an improved thermal conductivity in the perpendicular direction prepared by orienting the molecules of a polymer having high thermal conductivity in the vertical direction by applying a magnetic field.
However, the techniques of forming a pattern using a printing method to apply a solution of the organic good conductor and/or organic semiconductor on a substrate have several drawbacks.
First, although the applied organic material solidifies on the substrate as the solvent evaporates, this does not ensure that the entire region of the solution will always form an integral single crystal region or that the applied solution will always enter a homogenous polycrystalline state. The crystallinity and the domain size of the polycrystals may vary at different locations, and there may be amorphous regions mixed in the crystals. In such a case, the required performance of the good conductor and/or the semiconductor would not be achieved. This problem becomes more serious as the substrate size and the area onto which the solution is applied increase.
In the case where a transistor or the like is formed by disposing organic semiconductors between microgap electrodes, it is advantageous or essential to have the molecules of the organic semiconductors oriented in parallel to the direction of the gaps between the electrodes. To achieve this, it is necessary to subject a substrate to some kind of treatment for regulating the orientation direction prior to the organic semiconductor device fabrication or to apply an external force, such as an electric field or a magnetic field, during the fabrication of the organic semiconductor.
There are also some problems in the process of forming an oriented structure having a particular degree of orientation or higher.
For example, according to the technique in which a substrate to which the orientation-regulating capacity is imparted in advance is used, it is difficult to conduct a treatment such as rubbing on the substrate with electrodes preliminarily formed thereon.
According to the technique of applying an external force during the formation of a polymer assembly, the external force interacts with the crystal nuclei of the polymer during formation of the assembly and thus limits the direction of the growth of the crystal nuclei, resulting in formation of an oriented structure. In order to form an assembly with a high degree of orientation, these crystal nuclei must be grown in an adequate manner. However, it is possible that the conditions that are optimal for crystal growth are not optimal for achieving the desired orientation.
According to the technique of applying the external force to an unoriented structure, the unoriented structure may rupture, and the stereostructure of the molecule itself may be irreversibly destroyed if the external force is excessively large.
Even if all of the problems described above are overcome, there is still a problem. In an actual production plant, an unoriented structure is stretched and rolled to form a film- or strip-shaped uniaxially oriented structure. If tensile force is used as the external force, a large-scale facility for uniformly applying large force becomes necessary, but such a facility may not necessarily have a capacity to stretch all types of materials without rupture.
In the case of forming an oriented structure by applying external force to an unoriented structure on the substrate, a substrate having a surface that promotes rotation and orientation of the polymer molecules by application of external force must be used. For example, a material that rarely bonds with the polymer chemically or electrostatically needs to be selected, or a substrate free of irregularities that would obstruct movement and rotation of the molecules must be prepared. Due to this requirement, problems arise if an electronic device is made by integrating the oriented structure and the substrate. For example, the electrodes on the substrate may obstruct formation of the polymer assembly. Etching operation for forming the electrodes may roughen the surface of the substrate and the degree of orientation in the portion where the oriented structure contacts the electrode and very close to the substrate would not be sufficiently increased. As a result, the device would not achieve the expected performance.
In making an oriented structure containing a plurality of types of polymers, polymers must be dissolved and mixed in the same solvent. Thus, a combination of polymers not easily miscible with each other, such as a combination of a hydrophobic polymer and a hydrophilic polymer and a combination of a soluble polymer and an insoluble polymer, cannot be used.